It is known to produce fire-retardant resins by curing various types of epoxide resins with phosphoric acid. U.S. Pat. No. 2,541,027 discloses resinous compositions prepared by reacting phosphoric acid or monoalkyl dihydrogen phosphates with epoxides containing at least 6 carbon atoms, one or more ethereal oxygen atoms and having a 1,2-epoxide equivalency greater than one. The disclosed compositions are said to have utility as protective surface coatings, particularly when applied to ferruginous metals.
It is also known to prepare resins from epoxides containing methylol-substituted benzene rings. U.S. Pat. No. 2,965,607 discloses the preparation of viscous liquids, having utility as plasticizers, by polymerizing the methyl ether of 2-epoxypropyl-4,6-dimethylolphenol ##STR1## with aluminum or stannous chloride catalyst. No use of phosphoric acid in any role with the starting phenol or with the resulting polymers is disclosed.
U.S. Pat. No. 3,925,315 discloses the preparation of rapid curing, epoxide pre-polymers having the formula ##STR2## wherein m and n each have a value of at least 1. (The '315 patent also discloses that methylol-substituted polyether epoxides such as ##STR3## are known but cure less rapidly.) The methylolepoxides claimed in the '315 patent are said to have utility as adhesives, protective coatings and as encapsulants for electrical components. Again, incorporation of phosphoric acid is not disclosed.
It has not been suggested previously that useful resins might be prepared simply by reacting methylol- (or alkoxymethyl) substituted epoxides with phosphoric acid, or even that such reactions should be attempted.
The prior art is particularly deficient in not recognizing that thermosetting (and otherwise desirable) resins can be obtained by reacting mono-functional epoxides with phosphoric acid, provided that at least one methylol or alkoxymethyl group is present in the epoxide molecule.
The failure of prior workers to realize the advantages of, or to prepare, reaction products of phosphoric acid with epoxides containing methylol or alkoxymethyl groups may well be due to the fact that the latter types of groups are themselves known to be quite reactive. That is, reaction of the oxirane group(s) and of the methylol (or alkoxymethyl) groups with phosphoric acid (or with each other) would be expected to occur. In fact, it has been found that anisyl alcohol (p-methoxy benzyl alcohol) reacts very readily with phosphoric acid in dioxane solution and at room temperature to form a mixture of products which can be shown by infra-red and nuclear magnetic resonance spectroscopic analyses to contain no alcoholic hydroxyl groups.